TL;DR: It's not about pushing Windows 10, it's about pushing the Windows Store, because Steam is the real threat to Windows.
I don't have much to say about the S or the Scorpio, but I have a giant fuckload to say about bringing all their exclusives to Windows 10. It's not so much about just pushing Windows 10. Windows 10 is doing just fine. MS has had the OS market locked down for decades. PC gamers are going to run Windows, because although Linux gaming is getting substantially better (more on that later), Windows is still really the only game in town. They've already handed out free upgrades, and no new off-the-shelf PC is going to ship without it.
What this is really about is the Windows Store and UWP. Right now, the overwhelming majority of the PC gaming market is going through Steam, and that's a whole lotta sales MS isn't getting a cut of. Trouble is, not many publishers are interested in selling through Windows Store, and those that are are sure as hell not interested in selling there exclusively. Steam is the de facto standard for PC game distribution. If you want sales, you gotta be on Steam. Add to that that gamers have been less than impressed with the UWP versions of games, and the situation is looking dire. So dire that they are willing to jeopardize the XBox brand in order to "encourage" gamers onto their platform. This is not them taking on Apple and Linux. this is them taking on Valve.
Now while my gut instinct is to say that it's one more case of MS showing up late to the game and using questionable business practices and market dominance to tilt the field in their advantage, that's not the case here for two reasons. Firstly, nothing they have done yet is all that shady. It's bad news for the XBox brand, no question, but as long as they're undermining their own divisions and not competitors, more power to them. Secondly, they don't have market dominance, in either consoles or PC game distribution.
So, what will the future hold for their Windows Store strategies? Probably anything they can get away with to increase their market share. They will almost assuredly start throwing more money at publishers to buy exclusives, whether timed or permanent. That might be a tough sell for the publishers, but MS does have basically all the money. Will they go so far as to decree that if you want to release your game on both XBox and Windows 10 you have to release solely through the Windows Store? Maybe, but I don't have the market clout to go that far right now. Will they require an XBox Live subscription to play their exclusives online? I'd call it 50/50. That's probably more of a "Phase 2" strategy, for once they've taken a big enough bite out of Steam's pie. They'll surely try to sell it as the "better" way to game online, because features, but I doubt they can force it right away.
Will they succeed at taking on Valve? I doubt it, but here's the thing. They have no choice but to try, because Steam is the only threat to Windows dominance right now. Not many people realize it yet, but they are backed into a corner, and the future is only going to get darker if they don't act fast Now I can already hear you laughing at this next part (by all means, feel free) but what has them really scared is Linux gaming. See? you're laughing. Now, maybe you haven't looked lately, but Linux gaming is starting to get entirely respectable. Civ5, X-Com 2, Borderlands, and more all play just as well as they do on Windows. Unreal and Unity engines can compile for Linux out of the box. The only thing holding gaming on Linux back is the reliance on OpenGL, which is frankly old and crusty compared to Direct X. But that is changing with the Vulkan API. If you don't know what that is, I encourage you to look it up. In short, it's a fully-modern cross-platform royalty-free 3D graphics API with wide industry support from the likes of Nvidia, Intel, and AMD. Doom (the new one) was just ported to Vulkan. Most developers have said it's lighter-weight, easier to work with, and just generally better than Direct X. And let me repeat, it's royalty-free.
Now, Valve has already fired shots with their Steam machines, and as porting to Linux becomes more and more of a no-brainer, then the Microsoft OS cash cow is going to be threatened. Gamers are almost entirely driving the market in new PC's these days. Businesses have left the constant upgrade cycle, having learned that the huge expense does not allow them to do anything they couldn't do just fine before. Same goes for your average internet surfer. There's just no advantage to new powerful PC's and new invasive versions of Windows. Only gamers need the latest, greatest rigs and DirectX 10. If you're able to play all the same games on a free OS, neatly packaged up by Valve, then they could potentially lose the only market still driving PC sales. That's why we're seeing the all-or-nothing strategy.
So, there you have it. That is why MS is willing to risk their console division. They have to if they're going to save their OS business. Is that smart? Well, i would say no, not when their only weapons are Halo, Gears of War, Forza, and whatever exclusivity they can buy from third parties. But you'd better believe they're gonna go full scorched-Earth with this, and if the XBox is a casualty, they're prepared to accept that. At any rate, it's going to be interesting to watch.